Hello Daniel,
Have you raised it with SAP ? It could be anything from a bug to the fact that you accepted non-severe errors in XPRA_AIMMRG phase (though chances of that are really less but there is different solution that accepting non-severe errors if you got the error which I am thinking you got).
Re-run this phase and then when you get the error just check at OS level for the all files with the time stamp near to error time and publish them here.
Regards,
Ruchit